Processes developed for the application of AEPs are used particularly with newborns and infants for whom the usual subjective audiometry (sounds of different frequencies are applied through headphones--the patient is to indicate whether he/she hears the applied sound) is not suitable. In this case the auditory brainstem responses (ABRs) are preferably used.
Generated by a single acoustic stimulus an ABR has a very small amplitude. It cannot be detected in the spontaneous EEG on the usual lead from the scalp through electrodes as the signal-to-noise ratio is very bad. In order to enhance the signal-to-noise ratio the so-called averaging method is employed. In this method a large number of stimuli is applied in a short period of time and the post-stimulus segments of the spontaneous EEG (sweeps) are summed (averaged).
Provided the interfering spontaneous EEG has a random character its proportion in the result of the averaging becomes smaller and smaller with progressing summing.
Averaging must be continued until an AEP is present that can be evaluated reliably. The decision whether a response potential is present or not, is made by the examining person. The ostensibly objective audiometry is thus only objective concerning the recording of the data, but the evaluation is of a subjective nature.
This is the main problem of the objective determination of the hearing threshold by AEPs: On approaching to the hearing threshold the AEPs are becoming continuously smaller, the signal-to-noise ratio is hence becoming worse and worse. As a consequence, the evaluation is becoming problematic. Making the evaluation still more difficult is the fact that the spontaneous EEG, as a rule, is not an ideal stochastic process as it has been taken for granted. For that reason, the averaging result of the near-to-threshold AEP is superimposed by a substantial residual noise. The evaluation requires therefore a great deal of experience. Often all experience is no help because a random waviness in the AEP time range cannot be differed in the averaging result from an actual AEP or a low-amplitude response is completely covered by residual noise.
An effective support in decision-making can only be expected from suitable statistical methods that are not applied to the averaging result but to the sample of sweeps. The statistical method may be applied either to the time functions of the sweeps or to binomial features derived from the time functions or, after a spectral transformation having been made, to the spectra of the sweeps.
The development of a suitable process using a statistical (automatic) AEP detection makes the "objective audiometry" into a really completely objective determination of the hearing threshold.
A number of authors have published on that approach but the problem has not yet been solved satisfactorily.
The only practical result known so far is the U.S. Pat. No. 4,275,744 by Thornton and Obenour and the device ALGO-1 Plus from the company NATUS, which operates according to this method.
The method described in that patent relates to the middle latency responses (MLRs) whereas the ALGO-1 Plus device employs the ABRs that are more suitable for the hearing test of babies both methods using, in principle, the same process: The basis is a model ABR, the so-called template, as it is expected of normally hearing infants as the average response to the acoustic click stimulus used. The template is here the average of the ABRs of 35 normally hearing babies to a stimulus of 35 dB HL. Based on this template 9 data points were determined which are particularly stable. Only the sweep values at these points are then evaluated with different weights using a binomial statistics. At the selected times the template is positive or negative respectively after the stimulation. Now it is checked for single sweeps whether the amplitude is, at these points, also positive or negative respectively. The number of agreements is counted and a test is made using a statistical method, whether it is greater than for a purely random signal.
As long as no AEP is detected by the statistical test, the stimulation is continued, i.e., the number of the sweeps included is increased until either a response is evident or a given number of sweeps (here 15,000) has been reached and the test is stopped giving the conclusion: No AEP detectable.
The critical drawback of this process is the orientation on a model potential. Using such a template, the relatively large inter-individual variation range of the ABR time functions cannot be considered. This results in a relatively high probability that an existing potential will not be detected because its shape deviates from the model given. A pure time shift of the response potential (latency variation) is, however, intercepted by a search and adapting algorithm, which evidently is restarted every 500 sweeps.
Another disadvantage is due to the fact that the used statistical test is a so-called one-sample test, i.e. only the conditions at one point in time are tested (in this case, at the points in time marked at the template). Since 9 time points of the sweeps are analyzed, there are 9 test results. The individual statistical test results may be contradictory and must be linked by an additional procedure to yield a single statement.
Based on the concept of a screening device, with this concept having been adapted to the process and this device being intended not to determine the hearing threshold but "sift out" hearing impaired children, the device uses only a fixed, relatively highly suprathreshold stimulation intensity of 35 dB HL. The result is only a yes/no answer (hearing impaired/not hearing impaired). The concept needs not be objected but the relatively high stimulation level chosen is critical because it evokes a detectable response even in the case of a considerable hearing impairment.
Otoacoustic emissions (OAE) are sound emissions of the inner ear, which can be recorded in the external ear canal using a sensitive microphone. OAE are generated by the outer hair cells. The outer hair cells are capable of active oscillatory contractions, which cause a very sharp frequency tuning of the basilar membrane in the inner ear. The oscillations of the outer hair cells release oscillation energy that can be measured as retrograde sound emission in the external ear canal..sup.1 FNT .sup.1 Sebastian Hoth, Thomas Lenarz: Otoakustische Emissionen, Grundlagen und Anwendungen (Otoacoustic emissions--Fundamentals and applications). Georg Thieme Verlag Stuttgart, 1993.
If OAEs exist, operational capability of the inner ear can be assumed.
Different kinds of OAEs are known.
Today, the most relevant OAEs are
the transient-evoked OAEs (TEOAEs) and PA1 the distortion product otoacoustic emissions (DPOAEs).
The OAEs can be measured relatively easily consuming little time (using commercial instruments), and are well suited to form the basis of a hearing screening test with babies and infants. Also here the problem is (just as with the AEPs) that the existence/non-existence of OAEs must subjectively be estimated by the observer. Also here, as with the AEPs, the averaging method is used to enhance the signal-to-noise ratio. A fixed predetermined number of averaging steps must be taken which yield an interpretable result even for very small OAEs. Inevitably, time is wasted for OAEs of larger amplitudes for they could already be detected with a smaller number of averagings. But a screening test should require a test period as short as possible.